Belgium Company going to China for the ISRS 2019 in Beijing

Good day. 5 of our employees are going to the ISRS 2019 Symposium in Beijing (Conference dates May 26-31). I understand that the visa application must be filled out, what should be checked on:
Section 2.1 Major Purpose of Your visit
Section 2.8 Information of the Inviter: is this the ISRS 2019 Symposium Conference, or do I need some kind of letter from the Symposium representative.
Does each of the employee's need something additional (like invitation letter) than just submitting the visa application, along with the passport, copy of passport, Pictures (how many) round-trip airline tickets, hotel reservations?
Thank you